U.S. Pat. Nos. 3,168,094 (Siltamaki) and 3,835,847 (Smith) disclose traction apparatus in which the traction is controlled by means of an adjustable spring-loaded lever responding to the tension in the drawing cable. This lever operates a switch when the desired traction has been reached, whereby the motor is switched off, and a timer is actuated to reverse the motor after a predetermined delay. An arrangement of this kind has the disadvantage that no correction is possible for an excessive traction magnitude, due for instance to a shift of the position of the patient.
U.S. Pat. No. 3,710,787 discloses a similar arrangement wherein the lever is constituted by the motor, which is pivotably suspended. In order to prevent an excessive traction magnitude, the disclosure relies on a limitation of the current that may be supplied to the motor.